Rectangular art tiles, but turning them into a radial level design. That's just so smart. The only thing I'd like to see more is a little variation in the planets. Even just modulating the color of the grass and trees, or maybe using different sprites as "foliage" for the planet. It would have made the galaxy even more alive!
This was a neat little puzzle with a really endearing story line to keep the player engaged between levels. Such a pleasure!